It is well recognised that fast and accurate fault determinations for electronic circuits involving component failures or degradation is important. Electronic components and/or circuits or other elements present on circuit boards may fail in the field due to damage, age or otherwise degrade sufficiently that they no longer meet the manufacturer's specifications. The failed circuit boards are tested to determine the source of the failure, which typically will be one or more components, electrical connections or other elements on the board.
Following the fault diagnosis, attempts are typically made to repair the board. Standard test/diagnostic practices for troubleshooting a circuit board include the use of various testing devices which are designed to identify one source of the failed or marginal component, other circuit element or connection. These test devices include the use of benchtop equipment involving either automatic or manual comparison of digital signatures of circuit components while the circuit is operating. Such a device is shown and described in U.S. Pat. No. 4,965,516. Such devices include probes which physically contact circuit elements, produce a signal signature, usually in response to a signal stimulus, and then compare that signature with a standard or reference signature. The signature can be in analog or digital format.
These test/diagnostic devices are best suited for circuits operating below 100 MHz. At significantly higher circuit frequencies, i.e. 200 MHz and above, including most RF (radio frequency) circuits and microwave circuits, such devices are usually not effective. Any contacting circuit element, such as a probe, used to obtain signals from such an operating circuit is likely to produce self-generated failures in the circuit or a distortion in the signature. Thus, conventional fault determination test devices and techniques are typically not useful with high frequency circuits.
Hence, it is desirable that test equipment and techniques be developed which are capable of making reliable and accurate fault determinations in high frequency circuits, without introducing errors into the determination.